Mayor de Blasio’s unusual bid to buoy private-sector school bus workers with taxpayer funds is “ill-advised” — and City Council should reject it, a top fiscal watchdog said Wednesday.

Hizzoner is pushing for the city to shell out as much as $42 million to restore the salaries of veteran yellow bus drivers, matrons and mechanics whose salaries were reduced under contracts secured by the former administration.

Those contracts, covering just a portion of the roughly 7,700 public school routes, were expected to save taxpayers at least $210 million over five years starting this fall.

“This is an ill-advised measure that should not be adopted,” said Citizens Budget Commission President Carol Kellermann. “This type of payment would undermine the integrity of the City’s regular bidding and contracting process and set a troublesome precedent for other vendors that provide services to the City to seek similar de facto enhancements to their contracts.”

Several Council members argued at a hearing Tuesday that the mayor’s proposal would reward private bus firms that won routes by slashing veteran salaries by as much as half.

Those firms would now be able to split the cost of veteran workers with the city in the coming school year.

By contrast, firms that submitted bids with regular wage levels were iced out of the lucrative Department of Education contracts last year.

“We are correcting an unfair and ill-advised policy that slashed the wages and benefits of hardworking school bus workers, to the detriment of them and all the families who rely on our school buses every day,” said City Hall spokesman Wiley Norvell. “We have a tremendous amount riding on the quality and experience of the men and women who look after our children.”

Kellerman also expressed concern about where the money would come from, given that the city has already adopted its fiscal 2015 budget.

“The savings achieved in the current bus service contracts have already been incorporated in the City’s financial plan to fund other municipal services, which will have to be reduced in order to pay for the new payments to the bus companies,” she said.

Despite concerns, the Council is expected to approve the measure at a vote on Thursday.